The best evidence indicates that high blood pressure does not cause headaches or nosebleeds, except in the case of hypertensive crisis, a medical emergency when blood pressure is 180/120 mm Hg or higher. If your blood pressure is unusually high AND you have headache or nosebleed and are feeling unwell, wait five minutes and retest. If your reading remains at 180/120 mm Hg or higher, call 9-1-1.  
One side effect of diuretics is a loss of potassium, which is carried out of the body in urine along with the sodium. Potassium is needed for proper muscular movement and a deficiency of this mineral can result in fatigue, weakness, leg cramps, and even problems with the heart. So often, patients on traditional diuretics will be advised to take their medication with a potassium-rich food, such as orange juice or a banana, or they'll be prescribed a potassium supplement.

Medicines are available if these changes do not help control your blood pressure within 3 to 6 months. Diuretics help rid your body of water and sodium. ACE inhibitors block the enzyme that raises your blood pressure. Other types of medicines— beta blockers, calcium channel blockers, and other vasodilators—work in different ways, but their overall effect is to help relax and widen your blood vessels and reduce the pressure inside the vessel. [See also the free government publication “Medicines to Help You: High Blood Pressure” (PDF) from the US Food and Drug Administration.]

Stress leads to temporary elevations of blood pressure, but there is no proof that stress causes ongoing high blood pressure. Stress may have an indirect effect on blood pressure since it can influence other risk factors for heart disease. People who are under stress tend to engage more in unhealthy habits like poor nutrition, alcohol use, and smoking, all of which can play a role in the development of high blood pressure and heart disease.
Unlike high blood pressure symptoms, which are poorly defined and often totally absent, low blood pressure symptoms tend to be more upfront and easily recognizable. The development of symptoms is often a warning sign of a potentially serious underlying disorder. Generally speaking, your blood pressure would need to fall pretty dramatically before symptoms develop.